1. Field of the Invention:
The present invention relates to the separation and purification of salicylic acid, and, more especially, to the precipitation of salicylic acid from its sodium salt and the purification thereof.
2. Description of the Prior Art:
Salicylic acid is conventionally prepared by carboxylating sodium phenate wih carbon dioxide, under a pressure generally greater than 50 bars and at a temperature on the order of 150.degree. C. to 160.degree. C. Compare French Pat. No. 1,122,915, for example.
The sodium phenate is preferably employed in suspension in free phenol, for example, in a ratio by weight of phenol/sodium phenate of 3/1 to 5/1.
When the reaction is complete, the reaction mass is treated in order to separate therefrom the sodium salts of product hydroxybenzoic and hydroxyphthalic acids, which are essentially monosodium salicylate, possibly disodium salicylate and, in much smaller proportions, sodium parahydroxybenzoate and sodium 4-hydroxyisophthalate in this case.
The sodium salicylates obtained by the processes of carboxylation of sodium phenate are generally monosodium salicylate (monosodium 2-hydroxybenzoate), possibly containing some disodium salicylate. In the description that follows, except where otherwise indicated, the term "sodium salicylate" will refer more particularly to monosodium salicylate, possibly containing some disodium salicylate, but it will be understood that the process of the invention is not limited to the monosodium salt alone and can be applied equally as well to disodium salicylate, or to mixtures of monosodium and disodium salts principally comprising the disodium salt.
This treatment may especially include addition of water, which dissolves the sodium salt of hydroxybenzoic and hydroxyphthalic acids, and a liquid/liquid extraction step using a water-immiscible solvent. Such an operation thus provides an organic solution especially containing free phenol and an aqueous solution containing the sodium salts mentioned above.
Serious need thus exists in this art for the improved precipitation and purification of salicylic acid from such aqueous solutions.
Indeed, the treatment of aqueous solutions of sodium salicylate typically includes precipitating salicylic acid and the other organic acids with a strong inorganic acid, and especially with sulfuric acid. This treatment requires a large excess of the strong inorganic acid and several successive washings with water in order to remove the maximum of the salt formed (most frequently, sodium sulfate). The salicylic acid obtained may then be crystallized once, or several times, to reduce the level of other organic acids contained therein.
The salicylic acid obtained by the usual process contains amounts of sodium sulfate, and to a lesser extent, para-hydroxybenzoic acid and 4-hydroxyisophthalic acid, which are still too high for certain applications of salicylic acid in the food industry or in medicine.